Cellular communication systems are systems in which user terminals, especially mobile stations, in a particular area known as a ‘cell’ or a ‘site’ are served by a base station, and wireless communications to and from each user terminal are sent via the serving base station. Wireless communications sent from a base station to one or more user terminals are known in the art as downlink communications. Wireless communications from user terminals to their serving base station are known in the art as uplink communications.
Each cell of a cellular communication system can be considered to have a shape which approximates to a circle with the base station of the cell at the centre of the cell. The exact shape of each cell depends on the location of other cells adjacent to the cell. In some cellular systems, cells are divided into two or more sectors and each cell is said to be ‘sectored’. Each sector is an area of the cell defined by an angle subtended at the serving base station of the cell. Communications between the base station and user terminals in different sectors of a cell may be provided by a plurality of directional antennas at the base station. Thus, each of the directional antennas is dedicated to provide communications in a given sector. The most popular form of sectoring in the systems which have been proposed in the prior art is to use three antennas at the base station giving three coverage sectors each subtending an angle of one hundred and twenty degrees at the base station. However, sectoring using other numbers of antennas at the base station, such as four or six, is also known.
Examples of systems which employ cell sectoring in this way include systems proposed to operate in accordance with the 802.16e standard of the IEEE (Institute of Electrical and Electronic Engineers). The 802.16e standard of the IEEE, herein referred to as the ‘802.16e standard’ is an amendment to the 802.16 standard of the IEEE, herein referred to as the ‘802.16 standard’ to extend its applicability. The 802.16 standard entitled ‘Air Interface for Fixed Broadband Wireless Access Systems’ is the standard which was published by the IEEE on Apr. 8, 2002. It was developed by the 802.16 Working Group of the IEEE working on fixed broadband wireless access in Wireless Metropolitan Area Networks (WMAN). The 802.16 standard defines fixed terminal, point-to-multipoint communications by BWA (Broadband Wireless Access). The 802.16e standard is the standard which was published by the IEEE on Feb. 28, 2006 and is entitled ‘Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands’. It extends operation of the 802.16 standard to wireless broadband connectivity by mobile terminals. The expression ‘802.16e standard’ as used herein includes any amendments or successions to the 802.16e standard published by the IEEE (or any standards authority succeeding it) subsequent to Feb. 28, 2006.
Operation according to the 802.16e standard involves use of a form of OFDM modulation as an operating protocol to communicate information between terminals. OFDM (Orthogonal Frequency Division Multiplexing) is a spread spectrum technology which has been identified as one of the prime modern schemes for wireless networks. It allows high speed transmission of data via multiple lower speed sub-channels provided by division of the allocated frequency spectrum into sets of modulated sub-carriers.
The form of OFDM used in the protocol defined in the 802.16e standard is OFDMA (‘Orthogonal Frequency Division Multiple Access’). An OFDMA system is one in which different user terminals use the same frequency spectrum and each of these user terminals occupies a separate sub-channel defined by a specified sub-set of the sub-carriers.
In OFDMA communications, the available communication resource can be considered as a two dimensional entity and can be represented graphically by a two dimensional map or grid. One dimension represents frequency and the other dimension represents time. Referring to the frequency dimension, the OFDMA sub-carriers are pseudo-randomly spread on the entire available frequency spectrum of a given carrier for achieving frequency diversity. A designated group of spread sub-carriers is known as a frequency sub-channel. The time dimension is numbered (counted) in units of symbols, known also as OFDMA symbols. A given number of symbols in the time dimension makes up a frame.
In an OFDMA system, the communication resource available for use by each user terminal is defined in terms of a specified time in which the user terminal occupies a specified sub-set of the sub-carriers on a specified carrier frequency.
In accordance with the 802.16e standard, the frequency domain axis of the two dimensional resource representation is indexed in terms of sub-channels which are groups of sub-carriers. The time domain is indexed in terms of couples of (groups of two) OFDMA symbols in downlink transmissions and triplets of (groups of three) OFDMA symbols in uplink transmissions. The minimum communication resource unit specified by the 802.16e standard is a unit rectangle known as a ‘slot’. In downlink transmissions a slot has dimensions of one sub-channel along the frequency domain axis by one OFDMA symbol couple along the time axis. In uplink transmissions, a slot has dimensions of one sub-channel along the frequency domain axis by one OFDMA symbol triplet along the time axis. Communication is in the form of a continuous series of frames. In one possible mode, the TDD (time division duplex) mode, each frame includes a downlink sub-frame followed by an uplink sub-frame on the same carrier frequency. In another possible mode, the uplink and downlink transmissions are on different carrier frequencies and each of these transmissions comprises a sequence of consecutive frames
In the TDD mode, each of the downlink and uplink sub-frames for each carrier has a fixed total number of sub-channels and a fixed total number of OFDMA symbols available. Resource allocations within each sub-frame are generally selected by the base station. Allocations in accordance with the 802.16e standard are selected as groups of adjacent slots. There are two possible forms of such groups employed. These are known as rectangular and linear groups.
In rectangular groups of slots formed in accordance with the 802.16e standard, each group of slots is formed of a rectangle with a minimum height of one sub-channel along the frequency axis and a minimum width equal to a given number of OFDMA symbols along the time axis. The given number of symbols is two for downlink sub-frames and three for uplink sub-frames. Rectangular groups are used for transmission of certain specified types of system control information.
In linear groups of slots formed in accordance with the 802.16e standard, each group is formed by using adjacent slots along a row of slots. When the end of the row is reached, the slots in the next row are used, and so on. This method of forming groups is also known in the art as a ‘raster scan’ method, since it resembles a raster scan used for example in line by line scanning of a video image. For downlink sub-frames in the TDD mode, the rows for raster scanning for linear groups can extend along the frequency axis. For uplink sub-frames, the rows for raster scanning extend along the time axis. These rows may extend to the boundary of the sub-frame. Linear groups of slots are used for transmission of data known in the art as ‘data bursts’. Data bursts include traffic information, that is user communicated information or messages, as well as system control information other than that which is specified to be transmitted in rectangular groups.
The 802.16e standard enables the partitioning of downlink transmission resources from a base station to give sectored coverage of user terminals such as mobile stations in each sector of the cell served by the base station. Such partitioning allows inter-sector interference to be significantly reduced. The method is known as ‘PUSC’ (partial usage of sub-channels) and allows the downlink sub-channels to be distributed to the sectors of a sectored cell evenly in segments in a mutually exclusive manner.
In contrast to the specifications for downlink transmissions, the 802.16e standard does not specify any method for the distribution between sectors of uplink communication resources for communications from user terminals to the base station. For single frequency TDD systems, if all sectors use all of the available uplink resources, significant inter-sector interference will be experienced at the base station and hence the efficiency (average number of bits per second per Hertz) in the uplink direction will be low. Therefore, it is desirable to use partitioning of uplink resources between the different sectors of each cell.
No method is known which gives efficient and adaptable distribution and allocation of resources to different sectors of a sectored cell in a cellular communication system, wherein communication resource slots have two dimensions in the frequency and time domains, e.g. according to an orthogonal frequency division multiple access protocol such as the protocol according the 802.16e standard. For example, in a known distribution method which has been proposed for use in a cell having three sectors, the sectors are allocated resources by dividing by three the number of rows (frequency sub-channels) of slots in each uplink TDD sub-frame and allocating the resulting number of rows to each of the sectors in turn. Using this method the rectangular group(s) of slots and the linear group(s) of slots allocated for a given sector are all in the same group of allocated rows for the sector. This method is not efficient for the following reasons. Usually the number of sub-channels is not divisible by three so it is not possible to make an equal division of rows. Furthermore, rectangular groups providing control channel allocations tend to be provided on some frequency sub-channels but not others. This can restrict the number of slots allocated to a given sector for one or more given frequency sub-channel rows. Furthermore, this known method is not flexible for large load variations between sectors.